1. Field of the Invention
The present invention relates to an image forming apparatus and, more specifically, to an electrophotographic image forming apparatus which is adapted to form an image by developing an electrostatic latent image formed on a latent image bearing member with a developer.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a copying machine or a printer, an electrostatic latent image corresponding to an image to be formed on a sheet-shaped recording medium is formed on a surface of a photosensitive member serving as a latent image bearing member. The image forming apparatus includes a developing unit which supplies a developer such as a toner serving as a colorant to the photosensitive member and causes the toner to selectively adhere to the electrostatic latent image for visualization of the electrostatic latent image formed on the surface of the photosensitive member.
A toner image formed on the surface of the photosensitive member through the development by the developing unit is transferred onto the sheet-shaped recording medium. Then, the recording medium having the toner image transferred thereon is passed through a fixing unit in which the toner image is heated to be fused. The fused toner image is fixed to the recording medium.
After the transfer, a part of the toner not transferred onto the recording medium remains on the surface of the photosensitive member. The residual toner is removed from the surface of the photosensitive member for the next image formation. Therefore, a cleaning unit which removes the unnecessary residual toner from the surface of the photosensitive member is provided adjacent to the photosensitive member. The removed residual toner is stored in a container of the cleaning unit.
Methods for the development of the electrostatic latent image with the use of a dry developer are broadly classified into the following two categories: a two-component development method which employs a two-component developer including a magnetic carrier and a toner; and a single-component development method which employs a single-component developer including a toner alone.
In the two-component development method, the two-component developer including the toner and the magnetic carrier is carried on a surface of a developing roller serving as a developer carrying member by utilizing a magnetic force of a magnet incorporated in the developing roller, and transported to a developing area opposed to the photosensitive member by rotating the developing roller. When the developer is transported to the developing area, the charged toner contained in the developer electrostatically adheres to the electrostatic latent image on the surface of the photosensitive member, whereby the electrostatic latent image is developed. A residual part of the developer passed through the developing area is fed back into the developing unit, and magnetically released from the surface of the developing roller to be thereby collected in a developer tank of the developing unit. For stable development in the two-component development method, a virgin toner should be additionally supplied to the developing unit according to consumption of the toner, so that the proportion of the toner in the developer, i.e., the concentration of the toner, is kept constant.
In the two-component development method, an agitating mechanism such as an auger mechanism is generally provided in the developer tank of the developing unit for triboelectrically charging the toner by friction between the carrier and the toner. Therefore, the two-component development method is less susceptible to stagnation of the developer in the developer tank, but the developer tank has some dead space in which the developer is not easily circulated. That is, the stagnation of the developer is liable to occur in a portion of the developer tank in which an agitating force is weak and, particularly, in a portion of the developer tank adjacent to an area at which the developer is released from the surface of the developing roller.
In the single-component development method, on the other hand, there is no need to control the toner concentration.
Also, since mixing the carrier with the toner through the agitation in the developer tank as needed in the two-component development method is not required, the single-component development method does not need to provide a complicated agitation mechanism such as an auger mechanism, but is merely required a simple mechanism for supplying the toner to the developer carrying member.
That is, the single-component development method does not require the carrier, making it possible to correspondingly reduce the volume of the developer tank and hence the size of the developing unit. And further, the single-component development method ensures easier maintenance.
On the other hand, since the agitating mechanism is not provided in the single-component development method, the developer is poorly circulated in the developer tank as compared with the two-component development method.
In order to prevent the poorer circulation of the developer in the developer tank, a developer container is proposed which is divided into two chambers communicating with each other and includes a screw for circulating the toner between the two chambers (see, for example, Japanese Unexamined Patent Publication No. HEI8(1996)-278698).
In view of an opposed relationship of the latent image bearing member and the developer carrying member, the following difference is observed between the two-component development method and the single-component development method.
That is, in the two-component development method, the latent image bearing member and the developer carrying member are spaced a predetermined distance from each other in opposed relation, and a height of a magnetic brush formed by the developer carried on the developer carrying member is restricted to a predetermined level greater than the aforementioned distance by a developer amount restricting member. The latent image is developed, while a distal edge portion of the magnetic brush is kept in sliding contact with the surface of the latent image bearing member.
On the other hand, the single-component development method includes a contact development method and a non-contact development method. In the contact development method, a layer of the developer carried on the developer carrying member is restricted to a predetermined thickness and it is brought into contact with the latent image bearing member for the development. In the non-contact development method, the developer layer carried on the developer carrying member is spaced a predetermined distance from the surface of the latent image bearing member, and it is opposed to the surface of the latent image bearing member through the distance for the development.
In either of the methods, a developing bias is applied between the developer carrying member and an electrically conductive support base of the latent image bearing member for causing the toner to selectively adhere to the latent image. The developing bias may include a DC component alone, or include a DC component and an AC component superposed one on the other.
Where a multi-color toner image is formed by superposing a plurality of images developed on a single photosensitive member by a plurality of developing units, there is a possibility that a toner image developed by one of the developing units is touched and disturbed by the other inactive developing units in the contact development method in which the developer carried on the surface of the developer carrying member is brought into contact with the latent image, regardless of whether the developer is the two-component developer or the single-component developer. For prevention of such a problem, there is proposed an arrangement such that the inactive developing units are temporarily retracted away from the latent image bearing member with the use of eccentric cam mechanisms (see, for example, Japanese Unexamined Patent Publication No. HEI9(1997)-274386).
For the visualization of the electrostatic latent image formed on the surface of the latent image bearing member, the developer carried on the surface of the developer carrying member is transported to the latent image bearing member to cause the developer electrostatically adhere to the electrostatic latent image. At this time, a part of the developer not adhering to the latent image on the latent image bearing member, i.e., a part of the developer not used for the development, is fed back into the developer tank by the rotation of the developer carrying member, then mixed with the developer in the developer tank, and carried again on the surface of the developer carrying member to be transported to the latent image bearing member, regardless of whether the developer is the two-component developer or the single-component developer.
It is ideal that the developer not used for the development but fed back into the developer tank is all released from the surface of the developer carrying member and then sufficiently mixed with the toner in the developer tank, and carried again on the surface of the developer carrying member to be transported to the latent image bearing member.
In practice, however, the developer is liable to stagnate below the developer carrying member and around the developer feeding member which feeds the developer to the developer carrying member. This makes it difficult to mix or replace the developer with a virgin developer.
More specifically, a great amount of the developer stagnates in the vicinity of the developer carrying member and the developer feeding member without replacement during a prolonged use, and is not sufficiently mixed with the virgin developer.
On the other hand, a part of the developer to be carried on the developer carrying member and fed to the latent image bearing member is generally present adjacent to the developer carrying member and the developer feeding member. Therefore, the developer present adjacent to the developer carrying member and the developer feeding member is more liable to be repeatedly transported by the developer carrying member and fed back into the developer tank without use for the development. For this reason, a ratio of the developer repeatedly transported and fed back into the developer tank without use for the development becomes more higher.
The developer present adjacent to the developer carrying member and the developer feeding member is repeatedly subjected to stresses and, therefore, degraded to have a reduced fluidity. This makes it more difficult to properly circulate the developer, resulting in a vicious circle.
The degraded developer has variations in electrostatic property, fluidity and agglomeration property as compared with the virgin developer. As a result, the developer carried on the developer carrying member and transported to the latent image has significant variations in electric charge amount, so that the electric charge amount distribution is broadened. Further, the physical fluidity is varied, leading to variations in development characteristics. This results in lower image density and image roughness.
The arrangement proposed in Japanese Unexamined Patent Publication No. HEI8(1996)-278698 promotes the circulation of the developer by the screw provided in the developer container, but is less effective for circulating the developer stagnating in the vicinity of the developer carrying member and the developer feeding member.
The arrangement proposed in Japanese Unexamined Patent Publication No. HEI9(1997)-274386 prevents the disturbance of the toner image developed by the one developing unit by retracting the other inactive developing units away from the latent image bearing member, but does not prevent the stagnation of the developer.